71,2 mg of baking soda dont give a kH 4 solution.
You need 75,82 mg Na2CO3 ad 1000mL
4 kH equals 71,43mg/L CaCO3.
Using 99,8 % pure anhydrous Na2CO3 is best, so recalculate that from the molar equivalent of CaCO3:
M Na2CO3 = 105,99
M CaCO3 = 100,087
71,43 x (105,99/ 100,087) = 75,64mg/L
Recalculate for purity:
75,64/0.998 = 75,8mg/L
The best way is to make a stock solution and dilute so you can make several batches.
Weigh off 757,9 mg Na2CO3 and dissolve to 100,0mL with MQ water (Class A measureing bottle) stir with a magnet for 1 hour. Add 10,0mL to a 1000,0mL Class A measuring bottle and fill with MQ water.
Depending of the quality of your glassware, you can dilute the Na2Co3 to 1000,0mL first, and add 100,0mL to 1000,0mL (10 x dilution). But usually the 1000,0mL measuring bottles has less accuracy.
Best regards - the R&D labtechnician
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